Trailer Tyre Sizes, Load Ratings & Correct Air Pressure (PSI ↔ BAR Converter)

A complete engineering guide to semi trailer tyre sizes, load indexes, ply ratings and recommended air pressures for different tyre configurations. Includes a built-in PSI ↔ BAR converter commonly used in tyre inflation charts.

Tyres Engineering
📅 Published on 2025-11-28 | ✍️ Semi Trailer News Engineering Desk

🚛 1. Common Semi Trailer Tyre Sizes

Semi trailers worldwide primarily use **22.5-inch wheel tyres**, designed for durability, fuel efficiency and high load capacity. Below is a comparison of the most widely used models.

Tyre Size	Typical Use	Notes
385/65 R22.5	Steering / Single axle trailers	Wide footprint, higher volume
385/55 R22.5	Mega trailers / Low platform	Lower diameter, more stability
315/80 R22.5	Heavy duty / Off-road	High load index
295/80 R22.5	Regional transport	Balanced performance
445/45 R19.5	Special low-height trailers	Low rolling radius

🛠 2. How to Read Tyre Size Markings

What does 385/65 R22.5 mean?

385 → Tyre width in mm 65 → Aspect ratio (% of width) R → Radial construction 22.5 → Rim diameter in inches

Why load index matters?

The load index defines how much weight a tyre can safely support. Transporters must match the tyre’s load index with axle loads (especially 13-ton axles).

🔧 3. Recommended Tyre Pressures (PSI ↔ BAR)

Correct inflation ensures fuel economy, longer tyre life and safer braking performance. Below are typical ranges for common semi trailer tyres.

Tyre Size	Recommended BAR	Recommended PSI
385/65 R22.5	7.5 – 9.0	108 – 131 PSI
315/80 R22.5	8.0 – 9.5	116 – 138 PSI
295/80 R22.5	7.5 – 8.5	108 – 123 PSI
385/55 R22.5	8.0 – 9.0	116 – 131 PSI

🔢 4. PSI ↔ BAR Converter

📌 5. Signs of Incorrect Tyre Pressure

• Underinflation: shoulder wear, heat buildup, higher fuel usage
• Overinflation: center wear, reduced grip, unstable braking
• Mixed inflation: dangerous sway at high speeds

🤔 FAQ: Trailer Tyre Selection & Pressure

Which trailer tyre size is most common?

The 385/65 R22.5 tyre is the global standard for most semi trailers.

How often should tyre pressure be checked?

At least once per week or before long-distance trips.

Can low pressure cause tyre blowouts?

Yes — underinflation increases heat and can cause sidewall failure.

Does higher BAR mean stronger tyres?

No. BAR is inflation pressure; strength depends on load index and ply rating.

📦 Summary

Trailer tyres must match the vehicle’s load requirements, operating conditions and road type. This guide and the built-in BAR ↔ PSI converter help technicians select the correct tyre and maintain optimal pressure for safe transport operations.

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